Wet oxidation of organics at high pressures and temperatures has been predominantly used in the field of waste management for applications such as removing pollutants from organic processing installations, pulp and paper operations, milk processing operations and the like. U.K. Patent Specification No. 706,686 discloses a wet oxidation process adapted to the above uses and is additionally usable to generate steam energy from low grade powder coals by the wet oxidation of slurry coals. The process involves introducing to a reactor at high temperature and pressure a waste stream and air which are mixed within the reactor. The organics in the feed stream react with the oxygen in the air stream to produce mainly carbon dioxide and water. The reaction is exothermic with the consequent generation of heat. Providing the reaction is controlled, organics in a liquid feed stream can be oxidized to produce harmless products for discharge into the environment. Many additional techniques have been developed in the area of wet oxidation systems such as disclosed in U.K. Patent No. 812,832 and U.S. Pat. Nos. 3,852,192; 3,920,548; 4,155,848; 4,174,280; 4,217,218 and 4,229,296.
The wet air oxidation of low grade solid fuels as proposed in U.K. Patent No. 706,686 is attractive because the world's high grade fossil fuels are gradually being depleted. The lower quality solid fuels, that is, those with higher sulfur and ash content are being used more widely for stationary thermal energy and electrical power generation. There are two major and costly problems associated with dry incineration of low grade fuels. The higher sulfur content of the fuel results in unacceptable sulfur dioxide emissions, and expensive and problematic flue gas desulfurization is required. Boilers tend to be more complex in dealing with the low quality solid fuels because continuous ash removal equipment must be used to handle the high volumes of ash generated during combustion. In the wet air oxidation of the low grade fuels the coal is slurried with water and pumped into a high temperature and high pressure reactor. Air or oxygen is injected continuously either into the feed line preceding the reactor or the reaction vessel. The oxygen reacts with the organic materials to produce carbon dioxide, water and heat. Advantages of wet air oxidation of the low grade fuels is that the sulfur is oxidized to sulfuric acid (or sulfate salts) and the ash is essentially solubilized in the acidic slurry. Heat of reaction is removed from the vessel in the steam and liquid effluents such as disclosed by the technique in U.K. Patent No. 706,686. The effluents can either be used directly or heat exchanged to produce clean steam.
The major difficulty with the approach disclosed in U.K. Patent No. 706,686 and many other of the above noted patents is that only very dilute fuel: water slurries can be tolerated due to physical-chemical equilibrium within the unit. The quantities of air and organics introduced to the reactor system must be monitored and controlled to avoid excessive temperature increases due to heat of combustion and avoid run-away reactions. The net result, therefore, is low energy recovery per unit volume of reactor vessel. Attempts have been made to provide a degree of control on the temperature of the reaction. U.S. Pat. Nos. 3,852,192 and 4,229,296 involve the use of heat exchangers in the primary reaction zone to provide for initial heat-up of the reactor system in the primary zone and to control subsequently to a limited degree the temperature in the primary reaction zone. However, both patents are directed to the treatment of the dilute fuel to water slurries which are well within the safety margin of avoiding a run-away reaction.